This disclosure claims priority from Japanese Patent Application No. 6-244,393 and is related to Japanese Unexamined Patent Publication 5-148,689, both of which are incorporated herein by reference.
1. Field of the Invention
The present invention generally relates to a method for preparing a plating film exhibiting excellent abrasion resistance, heat resistance, shock resistance, and adhesion strength.
2. Description of the Related Art
Methods for preparing a composite plating film on a surface of a base material by utilizing a composite plating solution are known in the art. According to such conventional methods, the composite plating solution is formed by dispersing insoluble particles such as alumina (Al.sub.2 O.sub.3) in a metal matrix of a metal plating solution. Composite plating films prepared from such composite plating solutions generally have improved plating properties (e.g., abrasion resistance, heat resistance, and shock resistance) compared to pure metal plating films. However, the composite plating films prepared in accordance with most conventional methods fail to exhibit sufficiently acceptable plating properties.
It has been discovered that the above-described problems relating to ineffective plating properties can effectively be overcome by preparing a composite film having a non-uniform concentration of insoluble particles across the thickness (i.e., between the surfaces) of the composite plating film. For example, abrasion resistance is improved by increasing the concentration of insoluble particles at the outer surface of the composite plating film. Further, the adhesive strength of the composite plating film to a base material (substrate) is substantially improved by decreasing the concentration of insoluble particles at the inner surface of the film. Therefore, it is desirable to have a composite plating film with a non-uniform concentration of insoluble particles across its thickness (i.e., a higher concentration near the outer surface). Such a result can be achieved by practicing a method for forming a composite plating film having a so-called "gradating function," in which the concentration of insoluble particles continuously and gradually changes across the thickness of the film (i.e., from the outer surface of the film to the inner surface).
One method for practicing the gradating function for preparing a composition plating film is disclosed in Japanese Unexamined Patent Publication No. Hei 5-148689. According to this conventional method, the concentration of insoluble particles as a variable of film thickness is controlled by adjusting the specific surface area of the insoluble particles in the metal plating solution. This method is premised on the principle that the quantity of insoluble particles to be deposited in a metal matrix can be increased by decreasing the specific surface areas of the particles.
However, this conventional method also possesses inherent disadvantages. For example, practice of this method requires that a large number of different plating solutions be prepared--i.e., for each of the different specific surface areas, a corresponding plating solution is required.
Accordingly, practice of this conventional method requires the acquisition of large-scale plating equipment. For example, for each solution, a separate and corresponding plating solution tank must be provided.
In addition, precise and constant supervision is necessary to properly select from and switch between the plating solutions to achieve a gradual and continuous gradating function. Notwithstanding such supervision, it is extremely difficult to gradually change the composition of insoluble particles in a gradual and continuous manner as a function of film thickness.